The invention relates generally to solar powered air conditioning systems and more particularly to control systems used to improve the operational efficiency of solar powered air conditioning systems.
The possibility of using solar energy to decrease the usage of electricity in air conditioning systems has been widely suggested. Of all the proposed technologies, solar-assisted air conditioning systems using absorption chillers have become one of the most appealing. Solar absorption cooling systems have been installed in different parts of the world to evaluate their feasibility and performance. These types of systems are described in:
Van Hattem, D., and Dato, P. A., Description and Performance of an Active Solar Cooling System, Using a LiBr-H2O Absorption Machine, Energy and Buildings, 1981, Vol. 3, pp. 169-196;
Yellot, J. I., Operation of an Active Solar Air-Conditioning System in a Hot, Dry Climate, ASHRAE, 1982, Vol 2;
Bong, T. Y., Ng, K. C., and Tay, A. O., Performance Study of a Solar-Powered Air Conditioning System, Solar Energy, 1987, Vol.39, No. 3, pp. 173-182; and
Meza, J. I., Khan, A. Y., and Gonzxc3xa1lez, J. E., Experimental Assessment of a Solar Assisted Air Conditioning System for Applications in Puerto Rico, Solar Engineering, 1998, pp. 149-154.
Each of the these references is incorporated herein by reference in their entirety.
Although these systems have demonstrated the feasibility of reducing electricity usage, they have not become commercially available or widely used. A number of factors have contributed to this delay. In order to provide a cost-effective product, the electricity savings must be sufficient to justify the cost of the overall system. In these systems, the solar collectors represent a significant portion of system cost. Thus, it becomes critical to maximize the use of energy gathered by the solar collectors to minimize the number of solar collectors required by the system.
In addition, the solar collectors are only able to gather energy during daylight hours. The actual energy collections peak at approximately midday, then fall off into the afternoon. To extend the operational hours of these systems, energy storage tanks are used. As with the collectors, it becomes critical to maximize the amount of energy accumulated in the storage tank. This operates to extend the working hours of the system.
According to one aspect of the invention, the operational hours of a solar powered air conditioning system are extended by a control system. The control system determines a desired flow rate through a set of solar collectors that operates to maximize the energy collection. The control system also determines when the energy collected by the solar collectors will contribute to the energy in a storage tank. When this determination is favorable, the controller directs flow from the solar collectors into the storage tank.
According to another aspect of the invention, a solar powered air conditioning system circulates a fluid through solar collectors. An associated controller regulates the rate at which the fluid circulates. The rate is increased when the solar collectors produce a greater increase in temperature of the fluid. The rate is decreased when the solar collectors produces a lesser increase in temperature of the fluid. The heated fluid is pumped through an absorption chiller. A refrigerant is also pumped through the absorption chiller and through a heat exchanger positioned within a building. The heat exchanger cools the ambient temperature of the building.
According to a further aspects of the invention, the fluid from the solar collectors is circulated though a storage tank, and then pumped to the absorption chillier The temperature of the storage tank is determined at a plurality of positions. The fluid circulated from the storage tank to the solar collectors is drawn from the storage tank proximate the position having a lower temperature than the other positions. When the temperature of the fluid from the solar collectors is not greater than the temperature in the storage tank, then the fluid is circulated through a closed loop that excludes the storage tank. Otherwise, the fluid is circulated through the storage tank to effect an increase in temperature. This control is performed by operation of a three-way valve.
According to still further aspects of the invention, a coolant is circulated between the absorption chiller and a cooling tower by operation of a pump. A refrigerant is pumped through the absorption chiller and through the heat exchanger without applying a load at the heat exchanger at start up. In this mode, the heat exchanger has negligible effect upon the ambient temperature of the building. After applying a load, the refrigerant is pumped through the heat exchanger for an extended period of time after circulation of the fluid through the solar collectors terminates.
According to another aspect of the invention, a solar powered air conditioner pumps a first fluid through solar collectors. The flow rate is directly related to the amount of energy absorbed by the solar collectors. The temperature of the first fluid is determined after passing through the solar collectors. The temperature of the first fluid in a stratified storage tank is also determined. The first fluid from the solar collectors is pumped to the stratified storage tank when the temperature of the first fluid after passing through the solar collectors is greater than the temperature of the first fluid in the stratified storage tank. In one mode of operation, the first fluid from the solar collectors is pumped to an absorption machine while the plurality of solar collectors absorb energy. In another mode of operation, the first fluid from the stratified storage tank is pumped to the absorption machine when the plurality of solar collectors absorb an decreased amount of energy. A second fluid is pumped through the absorption chiller and to a heat exchanger configured to reduce an internal temperature of a building.
According to another aspect of the invention, a solar powered air conditioning system includes solar collectors, a pump, thermocouples, an absorption machine, a shut-off valve, a controller and a cooling circuit. The solar collectors are positioned to absorb energy. The pump is coupled with the solar collectors and configured to circulate a fluid through the solar collectors. One thermocouple is positioned at the plurality of solar collectors to determine a temperature of the fluid after passing through the plurality of solar collectors. A proportional flow valve is coupled with the plurality of solar collectors and configured to control a rate of flow of the fluid through the plurality of solar collectors. The solar collectors are coupled with a storage tank. Another thermocouple is positioned at the storage tank and configured to determine a temperature of the fluid within the storage tank. The controller operates the shut off valve to direct flow of the fluid as between a circulating loop from the solar collectors to the storage tank and a closed loop that excludes the storage tank. This determination is made based upon the respective temperatures of the fluid through the storage collectors and the storage tank. A cooling circuit couples with the absorption machine and is configured to effect a decrease in temperature to the interior of a building.
According to further aspects of the invention, the shut-off valve is a three-way valve. The absorption machine is also coupled with a cooling tower. The controller is further configured to direct the proportional flow valve to effect a fixed temperature difference across the plurality of solar collectors.
These and other aspects of the invention will be better appreciated with reference to the drawings and detailed description that follows.